This invention relates to a process for effectively and completely removing heavy metals from aqueous solutions with iron hydrosulfite (ferrous dithionite). More particularly, this invention relates to such a process wherein heavy metals are reclaimed, hazardous sludge is eliminated and an effluent having very low biological toxicity is produced. The present invention is useful for treating heavy metal ion containing waste waters generated by industries such as metal plating, metal surface finishing or printed circuit board manufacturing.
Prior to the present invention, it has been generally accepted that plating waste metals removed from alkaline solutions as metal hydroxide sludges must be handled as hazardous wastes. Environmental Protection Agency (EPA) jurisdiction over these wastes is well established. When generated by an electroplating facility and shipped off site, such sludge materials are defined as categorical F006 hazardous waste. Transporting and receiving and processing of these materials, even for reclamation and recycling, are restricted to EPA or State licensed operators. Due partly to this, and also due in part to the low metal concentration in such sludges, high recycling costs are incurred that usually exceed the recoverable value of the metals.
Prior to the present invention, methods for producing ferrous dithionite (iron hydrosulfite) have been explored as a possible new way to make sodium hydrosulfite. Sodium hydrosulfite is manufactured by several methods and several hundred million pounds are used worldwide each year. It is mainly use for, 1) bleaching woodpulp for newsprint, 2) reducing textile vat dyes, and 3) reductive leaching of ferric oxide from kaolin clays. All these major uses for hydrosulfite are for whitening or enhancing the color stability of materials to which it is applied. Most iron compounds are black or dark colored, thus discouraging the use of iron hydrosulfite for any of these major applications of sodium hydrosulfite.
U.S. Pat. No. 4,076,791 discloses improvements in making iron hydrosulfite and converting it to sodium hydrosulfite. More than 90% of the iron must be removed and replaced by sodium in order to use the resulting solution for leaching kaolin. A large volume of iron precipitate is produced which absorbs and wastes a large portion of the hydrosulfite, causing this process to be uneconomical. Prior to the present invention, no commercial use for iron hydrosulfite had yet been developed and efforts to develop iron chemistry in connection with hydrosulfite were abandoned.
Ferrous sulfate has been used to stabilize a sodium dithionite solution. In Japanese Patent JP 54029897, a 2% solution of sodium dithionite was used to decolorize dyeing wastewater containing Prussian Brilliant Red H3B [23211-47-4]. Adding some ferrous sulfate to the ditionite solution improves the stability of the decolored wastewater solution. There is no mention of any interaction or involvement by heavy metals existing in this prior work relating to ferrous ion and dithionite ion.
Metallic iron has long been known to react directly with certain other metals that are dissolved in acidic aqueous solutions. The iron dissolves into the acidic solution and the other dissolved metal deposits a metallic layer on the surface of the iron. Referred to as metallic replacement or cementation, this characteristic of metals has commonly been used in the commercial extraction of copper from ores and acid leaching of mine tailings. After some time, the surface of the iron is so covered with the other metal that the iron becomes unreactive and the reaction cases.
U.S. Pat. No. 3,902,896 addresses this limitation and discloses the use of a soluble thiosulfate compound to aid the cementation of such metals as copper, silver, gold, and platinum group metals from aqueous solutions. The patent discloses that the cemented metal flakes off the base metal, exposing fresh surfaces. Two properties of thiosulfate limit its utility for this purpose. In strong acid solutions, thiosulfate decomposes to form sulfur dioxide and elemental sulfur, which is colloidal and coats all surfaces it contacts. Also, dilute thiosulfate solutions are very corrosive on ferrous alloys, particularly on stainless steel materials.
U.S. Pat. No. 3,634,071 describes the use of sulfur dioxide for reducing ferric ions contained in recirculated ore leaching acid solutions. Some improvements in the cementation of copper using metallic iron were observed as relating to decreased oxidation of the iron and copper metals by ferric ions. No reference is made to dithionite. At the high sulfuric acid concentrations noted, it is very unlikely that dithionite ion could exist.
U.K. Patent Appliation GB 125828 A, filed June 16, 1983 discloses a process for removing copper ion from solution by contacting the solution with steel wool under controlled pH conditions. The copper cements over the surface of the fixed bed of steel wool, converting only a small portion of the iron into copper. This process is commercially undesirable due to 1) the uneconomically low conversion of iron to copper, and 2) the high cost of steel wool, and 3) the high labor cost for handling the materials. The recovered copper has a lower recycling value due to the cost of processing required for separating it from the residual steel wool fibers.
Many other methods exist for removing heavy metal ions from aqueous solutions, and which are commonly practiced in the pretreatment of industrial wastewaters containing environmentally toxic metals. When dissolved heavy metal solutions are free of chelating agents, they can be effectively treated by simply admixing an alkaline or caustic compound to precipitate the insoluble metal hydroxide. Sodium hydroxide, soda ash, lime or magnesium hydroxide slurry are all used to do this.
Frequently however, complexing ammonium ions or chelating compounds such as the sodium salts of ethylenediaminetetra-acetic acid (E.D.T.A.) and other having similar properties are present. They occur as ingredients in the used plating baths, cleaners and brighteners drained into the wastewater. In such cases, it is necessary either; 1) to use a strong chemical that breaks the chelant-to-heavy metal bond and forms a stable, insoluble compound or complex of the toxic metals, or 2) to add a substance that exerts a stronger attraction for the chelant than does the toxic metal ion, to free the heavy metal to precipitate as an insoluble hydroxide. Processes of both types are currently practiced.
Sodium sulfide is used to effectively precipitate heavy metals. Its sole advantage is the extremely low solubility of most heavy metal sulfides. Most are capable of existing in the presence of even the strongest chelating agents. Undesirable aspects of using a sulfide process include the extreme toxicity of hydrogen sulfide gas, which can be generated by contacting the sulfides with strong acids. Also, metal sulfide precipitates are slimy and difficult to filter. Large quantities of flocculants and filter aids are used, generating large volumes of sludge and corresponding high disposal costs.
Sodium borohydride is a strong, water soluble reducing agent that has an advantage of producing a compact, semi-metallic sludge. There are several reasons for its not having broad acceptance for heavy metal removal in wastewater treatment: 1) it is very expensive, 2) precipitated metals easily reoxidize and redissolve in the presence of dissolved ammonia, 3) dangerous concentrations of potentially explosive hydrogen gas can accumulate in the space above a reaction using sodium borohydride, and 4) at times when pH is not controlled perfectly, reactions occurring at an elevated pH of 8 or higher give off toxic fumes of hydrogen sulfide gas, dangerous to workers and sensitive equipment.
Hydrazine is another strong reducing chemical capable of breaking metal ion bond to chelants. It is used to a limited extent for heavy metal removal. But like borohydride, it too is very expensive to use and it too can generate dangerous volumes of hydrogen gas when acidified. Hydrazine has also been placed on a list of chemicals suspected of being carcinogenic. This has been a major impediment to its industrial use.
Several compounds have been used that form insoluble metal complexes with heavy metal ions. All exert a stronger attraction to the metal ion than the chelants normally occurring with the metals in the wastewaters. Insoluble starch xanthate is one such material, reportedly effective at complete removal of dissolved metal from the water. Its drawback is its generation of huge volumes of sludge, which retains a high water content and costs the user a severe penalty for disposing of same as a hazardous waste.
Other such complexing agents have gained widespread use including sodium dimethyldithiocarbamate (D.T.C.), and sodium diethyldithiocarbamate (D.E.T.C.). These are fairly effective at completely removing the heavy metal ions from solution. However, D.T.C. products are quite expensive and generate quite high volumes of sludge which requires costly reclaiming in order to recycle the recovered heavy metal. The precipitate is light in density and difficult to gravity settle. The sludge often floats on the water and gives off a foul smelling odor that is characteristic of the D.T.C. products. In addition, the dithiocarbamate compounds exhibit acute biological toxicity toward aquatic plant and animal species. Sodium dimethyldithiocarbamate is also used as the active ingredient in several EPA registered pesticide products.
At the present time, strict biological toxicity standards are being enforced upon industries by municipal sewerage authorities. Effluent toxicity is measured by placing live specimens of plant and animal species in diluted samples of such treated wastewaters. Recent data indicate that interactions exist between very low concentrations of certain heavy metals such as copper and silver, and certain anions such as nitrate, which produce more toxicity than is attributable to each component by itself. The implication of these developments is that even lower levels of removal of heavy metal ions from industrial effluents is required. A costly evaluation of background toxicity factors is required when an industry's effluent fails to meet specific toxicity limits.
All chemical methods for removing heavy metals from industrial wastes and wastewater that are of practical use and in actual practice involve chemical reactions that precipitate the metals from alkaline solutions. Certain of these processes involve chemical reduction to metallic form and others produce metal compounds, either insoluble organo-metallic complexes or metal sulfide or hydroxide sludges. The sludges of all these processes are fairly soluble in acidic water and the heavy metals are rapidly redissolved if the material is exposed to strongly acidic water.
The conventional wastewater treatment process, perhaps most frequently used by the largest number of industries, uses ferrous sulfate heptahydrate powder. Ferrous ion is substituted at a controlled acidic pH of about 2 to 3, to replace toxic heavy metal ions that are bonded by chelating agents. This allows the heavy metal ions to be rendered insoluble as hydroxides which are precipitated from an alkaline solution.
In the presence of strong chelants or free ammonia dissolved in alkaline solutions, a large excess of this source of ferrous ion is required. Normally, 5 to 10 ferrous ions are added for each copper ion being removed from chelated wastewaters. In heavily chelated streams, as many as 25 to 30 ferrous ions per heavy metal ion may be required in order to prevent the chelants from dissolving the heavy metal hydroxide. The commercial ferrous sulfate has seven waters of hydration and is only about 20% iron by weight. In some cases, for example, over 100 pounds of ferrous sulfate powder is added to the wastewater for each pound of chelated or ammoniated copper removed, thereby generating 60 to 80 pounds of sludge.
In typical treatment systems, each additional pound of iron used adds about 4 pounds to the weight of sludge made. This can be reduced to about 3 pounds of dry sludge per pound of iron used if a sludge dryer is used. When ferrous sulfate is dissolved into wastewater, it causes acidity in the water. Each mole of iron introduced this way requires using two moles of sodium hydroxide to neutralize the iron and form ferrous hydroxide. Therefore, when large excess amounts of ferrous sulfate heptahydrate powder are used, the total chemical cost for treatment is compounded. Higher hazardous waste sludge disposal costs are also incurred.
Sodium hydrosulfite is a strong, water soluble reducing agent. It can reduce heavy metal ions to zero valence and produce a metallic precipitate that is resistant to reoxidation and redissolving in acidic solutions. Commercial products are available as either a 13-14% buffered solution or as 85-95% powder. However, sodium hydrosulfite solutions are quite unstable and have a very short shelf life. Storage tanks need to be refrigerated and inert gas blanketed. The powdered products have an acid odor and a dust that is extremely irritating to a worker's eyes and nose. Damp or wet powder can spontaneously ignite into flames, creating a toxic smoke of sulfur dioxide. These objectionable properties have prevented sodium hydrosulfite products from gaining any major share of usage for heavy metal removal or for wastewater treatment in general.
It would be highly desirable to provide a safe, simple, reliable and economical process for removing heavy metals from aqueous solutions that would;
1) yield a superior quality aqueous effluent that is low in biological toxicity and compliant with all regulations for discharging into a public sewer or waterway, and, PA1 2) eliminate producing an F006 hazardous waste sludge that is normally generated at an alkaline pH and usually has a low metal content, and, PA1 3) reclaim the metals in a concentrated metallic form that yields a net positive value when recycled, and, PA1 4) use readily available and economical materials that are non-hazardous and do not cause irritating or foul odors or explosive gases.